Method for forming a metal contact on a surface of a semiconductor, and device with a metal contact

ABSTRACT

A method is described for forming at least one metal contact on a surface of a semiconductor and a device with at least one metal contact. The method is used for forming at least one metal contact ( 60 ) on a surface ( 11 ) of a semiconductor ( 10 ) and has the steps of: applying a metal layer ( 20 ) of palladium onto the semiconductor surface ( 11 ), applying a mask ( 40, 50 ) onto the metal layer ( 20 ), and structuring the palladium of the metal layer ( 20 ) using the mask ( 40, 50 ), wherein lateral deposits ( 21 ) of the metal are formed on sidewalls of the mask by the structuring so that the mask is embedded between the deposits ( 21 ) and the structured metal layer ( 20 ′) after the structuring. Since the mask is conductive, it can remain embedded in the metal. The deposits and the mask form a part of the contact.

CROSS-REFERENCED TO RELATED APPLICATION(S)

This Application is a National Phase Patent Application of, and claimspriority to and the benefit of International Application NumberPCT/EP2014/078189, filed Dec. 17, 2014, which claims priority to and thebenefit of German Application Number 10 2013 226 270.1, filed Dec. 17,2013, the entire contents of all of which are incorporated herein byreference.

The invention relates to a method for forming at least one metal contacton a surface of a semiconductor and to a device with at least one metalcontact.

Semiconductors, for example of GaN, gallium nitride, are used in varioustechnical fields. An example is the use of epitaxial semiconductorwafers for manufacturing ridge waveguides for a laser diode.

Many of these applications require metal contacts on a surface of thesemiconductor. Here, resistance values of such metal contacts dependsensitively upon impurities and/or crystal defects and/or thestoichiometry of the semiconductor surface at the time of theapplication of the respective metal contact. The stoichiometry of thesemiconductor surface can change, particularly during the processing ofthe semiconductor.

In order to leave the resistance values of the metal contacts unaffectedtherefrom, a layer of the respective metal is deposited onto thesemiconductor surface before each other processing of the semiconductorand is structured subtractively by means of a mask. If the metalcontacts consist of a noble and chemically very inert metal, a dryetching method, for example sputter etching, reactive-ion etching (RIE),inductively coupled plasma (ICP) etching, or chemically assisted ionbeam etching (CAIBE), is required for this purpose.

In such dry etching methods, a part of the removed metal deposits againon the surface as well as on the surface and the edges of the mask inthe course of the redeposition. After removing the mask, the metalredeposited at the edges of the mask remains as a kind of fence. Thisfence is difficult to remove and can cause the formation of cavities inthe course of the deposition which, in turn, can affect the reliabilityof the component including the semiconductor by obstructing the heattransport.

The formation of the fences according to the prior art is exemplified inFIGS. 1 to 3. FIG. 1 shows the unstructured semiconductor 10 of GaN witha metal layer 20 of Pd, palladium, on a surface 11 of the semiconductor10. As shown in FIG. 2, a mask 30, for example a hard mask of SiN_(x),is applied. Then, the metal layer 20 is structured by means of sputteretching, for example with argon. Subsequently, the semiconductor 10 isstructured by plasma etching, for example with chlorine, using the samemask. Subsequently, residues of the mask are removed. There remains thestructure with deposits 21 which were deposited laterally at the maskand protrude upwards over the structured metal layer 20′ after theremoval of the mask, as shown in FIG. 3.

The removal of the mask is carried out wet-chemically using hydrofluoricacid, for example.

A method for forming a structured tungsten layer by means of a titaniummask is described in U.S. Pat. No. 5,176,792. EP 0 889 519 A2 isconcerned with an electrode structure for a capacitor. A platinumelectrode is etched by means of a Ti—Al—N hard mask, the redeposition ofthe platinum during the etching leading to the formation of transientside wall fences. U.S. Pat. No. 6,433,436 B1 illustrates the fabricationof a multi-level-interconnect structure in a combined etching process inone step. DE 10 2009 034 359 A1 relates to a palladium-based p-contactfor a light emitting diode, especially a nano-pixel LED based on GaN.

The object of the present invention is to provide a method by which areliably functioning metal contact can be formed on a surface of asemiconductor.

This object is achieved by the method according to claim 1. The methodis used for forming at least one metal contact on a surface of asemiconductor made of gallium nitride and comprises the steps of:applying a metal layer of palladium onto the semiconductor surface,applying a mask onto the metal layer, and structuring at least the metallayer using the mask, wherein lateral deposits of the metal of the metallayer are produced on the mask by the structuring so that the mask isembedded between the deposits and the structured metal layer after thestructuring. The method is characterized in that the mask is aconductive hard mask, wherein the structuring also structures thesemiconductor and comprises the following steps: sputter etching themetal with argon, and plasma etching the semiconductor with chlorine.

Since the mask is conductive, it can serve as a lost mask, and it is notnecessary to remove the mask and the lateral deposits so that no surfacesection of the metal contact is concave. The deposits and the lost maskform a part of the contact.

In an advantageous embodiment, the mask includes at least one conductivematerial different from the metal. In this case, the different materialcan form a lowermost layer of the hard mask. Further, the hard mask ofsaid advantageous embodiment includes a layer made of the metal which isarranged on the different material.

As a result of the structuring, the metal encloses the differentmaterial so that the conductivity of the contact is only minimallyinfluenced by the presence of the material in the contact.

The conductive material can include, for example, titanium, nickel orchromium.

The semiconductor can be an epitaxial semiconductor. The metal contactcan be a part of a ridge waveguide. The metal contact can also be a mesastructure on a p-side of a micro-pixel LED or a nano-pixel LED.

According to the invention, a device according to claim 7 with at leastone metal contact on a surface of a structured gallium nitridesemiconductor is further proposed. The device is characterized in thatthe metal contact comprises a conductive hard mask material on acorrespondingly structured palladium layer and between palladiumdeposits, wherein all surface sections of the metal contact are convexor flat. That is, no surface section of the metal contact is concave.

The invention is described below in exemplary embodiments with referenceto the associated drawings. In the drawings:

FIGS. 1, 2 and 3 show schematically different states of a semiconductorduring a method for forming a metal contact according to the prior art,and

FIGS. 4, 5 and 6 show schematically different states of a semiconductorduring a method for forming a metal contact according to an exemplaryembodiment of the invention.

FIG. 4 shows the unstructured semiconductor 10 of GaN with a metal layer20 of Pd, palladium, on a surface 11 of the semiconductor 10. Theunstructured semiconductor 10 of the example is an epitaxialsemiconductor, but the present invention develops its technical effectalso for other semiconductors and is not limited to epitaxialsemiconductors.

As shown in FIG. 5, a conductive hard mask 40, 50 which includes alowermost layer 50 of Ti, titanium is applied. Other conductivematerials different from palladium are Ni, nickel, and Cr, chromium. Anuppermost layer 40 which is arranged on the lowermost layer 50 alsoconsists of palladium. The uppermost layer 40 is optional and, in theexample, considerably thicker than the metal layer 20. In the example,there are only two layers present so that the uppermost layer 40 isarranged directly on the lowermost layer 50. However, further conductivelayers are possible in the sense of the invention. When only one layeris used, it is the more advantageous the more similar the electricalproperties of the different conductive material are to that of themetal. Particularly, the hard mask can also consist of the metal of themetal layer 20.

Then, the metal layer 20 is structured by sputter etching, for examplewith argon, i. e., is removed in the non-masked area. Here, theuppermost layer 40 is removed correspondingly, but since it is thickerthan the metal layer 20, a residue from the uppermost layer 40 remainswhen the metal layer 20 is already completely removed in the non-maskedarea.

The metal removed in the non-masked area and from the surface of theuppermost layer 40 deposits laterally at the structured metal layer 20′and at the layers 40 and 50 of the mask in form of deposits 21 of themetal.

Subsequently, the semiconductor 10 is structured by means of plasmaetching, for example with chlorine, using the same mask.

As shown in FIG. 6, the deposits 21 and the layers 20′ and 50 enclosethe different conductive material of the layer 40. The residues of thehard mask, i.e., the residues of the layer 50 and the layer 40, are allconductive and can be used as a part of the contact so that neither themask nor the deposits have to be removed. Thus, no surface section ofthe metal contact is produced which is concave. Additionally, the methodproposed according to the invention requires fewer method steps. Thecontact can easily be contacted via the surface of the residue of thelayer 40, and the cavity between the deposits 21 is filled so that it isnot necessary to remove the deposits 21. Thus, the device proposedaccording to the invention is more reliable and more functional.

Metal contacts in the sense of the invention can be used advantageouslyfor various applications. For example, the metal contact can be a partof a ridge waveguide. For example, it is also possible for the metalcontact to be a mesa structure on a p-side of a micro-pixel LED or anano-pixel LED.

In an exemplary embodiment, the metal contact comprises a conductivehard mask material on a correspondingly structured palladium layer andbetween palladium deposits, wherein all surface sections of the metalcontact are convex or flat. That is, in this exemplary embodiment, nosurface section of the metal contact is concave.

The invention claimed is:
 1. A method for forming at least one metalcontact (60) on a surface (11) of a semiconductor (10) made of galliumnitride, comprising the steps of: applying a metal layer (20) ofpalladium onto the semiconductor surface (11), applying a mask (40, 50)onto the metal layer (20), and structuring at least the metal layer (20)using the mask (40, 50), wherein lateral deposits (21) of the metal ofthe metal layer are formed on sidewalls of the mask by the structuringso that the mask is embedded between the deposits (21) and thestructured metal layer (20′) after the structuring, characterized inthat the mask is a conductive hard mask, wherein the structuring alsostructures the semiconductor (10) and comprises the following steps:sputter etching the metal layer (20) with argon, and plasma etching thesemiconductor (10) with chlorine.
 2. The method according to claim 1,wherein the mask includes at least one conductive material (50)different from the metal layer, and wherein the at least one conductivematerial (50) forms a lowermost layer of the hard mask and the hard maskfurther includes a layer (40) made of the metal layer of palladium whichis arranged on the at least one conductive material (50).
 3. The methodaccording to claim 2, wherein the conductive material (40) includestitanium, nickel or chromium.
 4. The method according to claim 1,wherein the semiconductor (10) is epitaxial.
 5. The method according toclaim 1, wherein the metal contact (60) is a part of a ridge waveguide.6. The method according to claim 1, wherein the metal contact (60) is amesa structure on a p-side of a micro-pixel LED or a nano-pixel LED. 7.A device with at least one metal contact (60) on a surface (11) of astructured gallium nitride semiconductor (10), wherein the metal contactcomprises a conductive hard mask material on a correspondinglystructured palladium layer (20′) and the conductive hard mask materialis between the structured palladium layer and between palladium deposits(21) that are on the sidewalls of the conductive hard mask material,wherein no surface section of the metal contact (60) is concave.
 8. Adevice according to claim 7, wherein all surface sections are flat.